In a classic experiment which was performed by Newton as early as the year 1730, light from a test lamp was shown on a white screen and viewed by an observed. An adjacent part of the screen was illuminated by three different light sources equipped to give light of widely differing colors, typically, red, green and blue. Interestingly, it was found that by adjusting the intensities of the three colored light sources, they could be made to reproduce a combined light on the screen which would match that of the test lamp. The importance of this basic experiment is that it teaches that the light from three different colored light sources, commonly referred to as the primary lights, can combine to reproduce a test color by a technique known as additive color matching.
Once color photography was introduced it was an easy step to take to note that by shining primary lights of different colors on the same piece of photographic media, an image of their combined color would be obtained which was different from the color of the primary lights themselves. Such a concept is disclosed in U.S. Pat. No. 3,741,649, issued June 26, 1973, to Podesta et al. However, Podesta discloses merely an empirical system whereby, through trial and error, light is projected through a multitude of combinations of filters onto photographic media which is then developed to discover what usable color, if any, has been obtained. By noting which filters were used, some rudimentary repeatability of results was attainable. However, the method disclosed by Podesta makes no provision for reproducing a randomly selected target color and makes no provision for duplicating such a target color when a light source having a different color temperature is used, or when filters are used which are of colors different from those already used.
U.S. Pat. No. 3,322,025 issued May 30, 1967 to Dauser, discloses a somewhat more sophisticated, but still empirical method which involves the production of a cylindrical color solid wherein each colored segment is identified by its hue, value and intensity. The color solid is generated, apparently, by varying the voltages to three primary light sources, red, green and blue, to thereby regulate their intensities and thus produce the various colored segments comprising the color solid. Repeatability is obtained by noting the voltage input or light output of each of the respective lamps needed to produce each colored segment in the color solid.
However, such a system is severely limited since it provides only for producing the cylindrical color solid by the use of three colored lights which together are able to form achromatic light. If it is desired to use three lights which are unable to form achromatic light, what technique to use is undisclosed. Further, if a different set of achromatic lights is used, it is clear that the first produced color solid is useless and that a new color solid must be prepared for each different set of lights employed. In addition, the method's ability to faithfully reproduce a color segment is questionable since it is well known that the color properties of photographic media vary from manufacturer to manufacturer and from lot to lot. How Dauser would compensate for such variations without regenerating a complete color solid for each batch of photographic paper used is not disclosed. In addition, it is well known that the color temperature of even standardized light sources changes with age and hence the colors they produce similarly change. Again, how such aging is compensated for, without the periodic reconstruction of a complete color solid, is a problem not addressed by Dauser. Further, it is well known that varying the voltage to a light source is a crude way to regulate its intensity since as the bulb dims or brightens, the color produced by the bulb also changes. Apparently Dauser failed to consider this variable which would tend to make his color solid nonlinear. Lastly, if both positive and negative photographic media were to be used, apparently a separate color solid would have to be generated for each, a costly and time-consuming process.
Many other attempts have been made over the years to systematically organize the various colors visible to the human eye. One of the most successful of the prior systems has been the CIE system (Commission Internal de L'Eclairage, or the International Commission on Illumination). As seen in FIG. 1, color as described in the CIE system may be plotted graphically in a plane Cartesian coordinate system wherein the x values are plotted horizontally on the abscissa 10, and the y values are plotted vertically on the ordinate 12. The results of such a plot is the standard CIE 1931 chromaticity diagram, generally designated at 14, showing a horseshoe shaped spectrum locus on which the points representing the chromaticities of the spectrum colors are plotted according to their wavelengths in nanometers. A straight line 18, upon which the purples and magentas are plotted, forms the base of the chromaticity diagram and connects the lower ends 20, 22 of the spectrum locus. The standard CIE diagram 14 may be obtained from recognized works on the subject such as the Handbook of Colorimetry by A. C. Hardy, published in 1936 by the Massachusetts Institute of Technology.
The CIE chromaticity diagram 14 is frequently used as a reference system or reference data base as in U.S. Pat. No. 2,850,563, issued Sept. 2, 1958, to Gretener. Gretener uses the CIE diagram 14 as a theoretical basis for explaining his method of more accurately reproducing images in color through the use of photography or television. Primarily, his method achieves accurate color reproduction by properly adjusting the spectral response of the recording apparatus. For reproducing flesh tones, the spectral response is adjusted in such a manner that the flesh tones are recorded with color components having an intensity of equal size, i.e., having a unitary ratio, or in the preferred case, with only the red and green components having substantially equal intensities.
In explaining his process, Gretener discusses the use of a so-called color triangle or filter triangle 24 plotted on the CIE diagram 14 which has the primary reproduction colors 26, 28, 30 plotted at its vertices. He notes that the color triangle 24 delineates the area on the CIE diagram within which a desired color may be reproduced by the additive mixture of a suitable ratio of intensities of the primary reproduction colors 26, 28, 30. Gretener further explains that if a color is to be reproduced by the additive mixture of three colored lights, the required ratio of intensities may be derived in a well-known manner from the location of the particular target color within the filter triangle 24 formed by the primary colors. It is notable that the thrust of the Gretener patent is directed towards a method of unitizing the ratio of at least two of the primary colors.
The Gretener patent, however, fails to disclose a method by which a target color may be produced on positive photographic media by calculating the exposure time needed for each of the primary colored lights. In addition, no method is disclosed by which the proper exposure times for each of the three primary light sources may be calculated such that when negative photographic media is exposed thereto, the target color will result on the developed media. Further, no indication is given by Gretener of how, when attempting to achieve a particular target color, the target color may be accurately located on the CIE diagram as a starting point for the calculation of the exposure time needed for each of the primary light sources.